Control device of induction motor

1. A control device of an induction motor which performs vector control on the induction motor, the control device comprising:

an excitation current command generation portion which Generates an excitation current command value based on a magnetic flux command value and which limits the excitation current command value based on an excitation current command maximum value;

a torque current command generation portion which generates a torque current command value based on a torque command value and which limits the torque current command value based on a torque current command maximum value;

a drive portion which supplies a drive current to the induction motor based on the excitation current command value and the torque current command value; and

a current command maximum value setting portion which sets the excitation current command maximum value and the torque current command maximum value,

wherein the current command maximum value setting portion changes, when the induction motor is accelerated/decelerated, the excitation current command maximum value based on which one of a first excitation current command maximum value and a second excitation current command maximum value is lower, and changes the torque current command maximum value based on the excitation current command maximum value and an allowable maximum current value of the induction motor or the drive portion,

the first excitation current command maximum value is based on the allowable maximum current value and

the second excitation current command maximum value is based on a power supply voltage of the drive portion, a current frequency of the drive current and an excitation inductance of the induction motor.

2. The control device of the induction motor according to claim 1 , further comprising: a determination portion which determines whether or not the induction motor is in an accelerated/decelerated state.

3. The control device of the induction motor according to claim 2 , further comprising: a magnetic flux estimation portion which estimates an amount of magnetic flux of the induction motor from the excitation current command value, a mutual inductance of the induction motor, a time constant of the induction motor and a time which elapses after the determination portion determines that the induction motor is in the accelerated/decelerated state,

wherein the determination portion determines whether or not the amount of magnetic flux estimated by the magnetic flux estimation portion is lower than the magnetic flux command value, and

when the induction motor is in the accelerated/decelerated state, and the amount of magnetic flux estimated by the magnetic flux estimation portion is lower than the magnetic flux command value, the current command maximum value setting portion changes the excitation current command maximum value and the torque current command maximum value.

4. The control device of the induction motor according to claim 1 , further comprising: a current frequency computation portion which computes the current frequency of the drive current from an actual speed value of the induction motor, a number of polar pairs or a number of poles of the induction motor and a slip frequency of the induction motor.

5. The control device of the induction motor according to claim 1 , further comprising: a voltage detection portion which detects the power supply voltage of the drive portion.

6. The control device of the induction motor according to claim 1 , further comprising: a storage portion which stores the allowable maximum current value of the induction motor or the drive portion and the excitation inductance of the induction motor.

7. The control device of the induction motor according to claim 1 , wherein the current command maximum value setting portion

determines the first excitation current command maximum value Idmax 1 by formula (1) below based on the allowable maximum current value Imax 1 and a constant K related to the vector control,

Idmax1=Imax1× K   (1),

determines the second excitation current command maximum value Idmax 2 by formula (2) below based on the power supply voltage V, the current frequency f and the excitation inductance L,

Idmax2= V /(2π f×L )  (2),

determines, as the excitation current command maximum value Idmax, which one of the first excitation current command maximum value Idmax 1 and the second excitation current command maximum value Idmax 2 is lower and

determines the torque current command maximum value Iqmax by formula (3) below based on the excitation current command maximum value Idmax which are determined and the allowable maximum current value Imax 1 ,

Iqmax=√(Imax1 2 −Idmax 2 )  (3).

8. The control device of the induction motor according to claim 7 , wherein the constant K related to the vector control is √(2).